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The purpose of this thread is to help focus the attention of the community on posts that may require reopen and undeletion votes. A request should be posted as an answer below (one request per answer).

Some guidelines:

  • Please be polite, and respect the many different viewpoints in our diverse community. This goes for the person making the request as well as those commenting on it.

  • There is a reopen queue. Please wait until a post has gone through this queue, before posting here. Notice that the first edit after the question is closed pushes the question into the reopen review queue if the edit is done within 5 days of closure, and so does a reopen vote. (If the review has already been finished, it is shown on the timeline of the question.) When in doubt, wait 24 hours after the last substantive action.

  • To inform readers of the current (and past) states of the targeted post, please add the information Reopened or Undeleted at the start once the request has resulted in some action. (If the action is undone, add this too, like Reopened, Reclosed.)

  • Do not only post a request, like "request reopening of link". Instead, make a case for your concern. Yet keep in mind that it can be easier to get your request handled if you try to frame it in a way that takes the feedback the post received into account positively rather then seeking confrontation. Also, try to improve the post before posting here.

  • In case of "small" requests, like one missing vote, it can make sense to ask in chat instead of posting here. The room CURED is a reasonable place for such requests. The same guidelines apply there.

  • If you are involved in the thread which you post about (e.g., you asked the question or you answered it), please disclose this.


Earlier versions of this thread that served as a model:

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    $\begingroup$ I edited the post inspired by a concern expressed in a comment. $\endgroup$
    – quid Mod
    Jul 19 '20 at 23:49

189 Answers 189

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Reopened

Please consider this question Some Combinatorics and Some Prime Numbers to be reopened. This was posted by me seeking for other solutions rather than mine. I think it's well explained.

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  • $\begingroup$ Please follow the guidelines in posting here. In particular this (quote): To inform readers of the current (and past) states of the targeted post, please add the information Reopened or Undeleted at the start once the request has resulted in some action. (If the action is undone, add this too, like Reopened, Reclosed.) $\endgroup$ Jul 22 '20 at 9:36
  • $\begingroup$ Okay, thanks! I got it. $\endgroup$ Jul 22 '20 at 9:39
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Undeleted


Please undelete Invariance of the Lagrangian vs invariance of its integral

It was recently autodeleted by Community because its net vote count was reduced to $0$ by a single vote cast in a recent downvote spree targeting me.

I'm still interested in the topic and am looking forward to an answer or two...

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Reopened

Please reopen: Given $\sinh x$, find the exact value of $\cosh x, \cos 2x$ and $\tan 2x$.

As the question stands now, there is evidence of clear and informative context, and through this, the OP has managed to answer their own question. The reasons for closing this question stem from when the question was in a worse state, but after the edits, there is nothing confusing, or anywhere in the question that lacks context.

The answers to this question are of good quality, so it would be nice to preserve the answers to this question, without forcing the OP to accept an answer.

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Reopened

Please re-open this question which is put on hold:

Understanding De/Suspension $\Sigma^{-1}(\Sigma{X})\neq X$

It was said that "unclear what you're asking" and people do not know the def of de-suspension.

However, the suspension is introduced earlier in the cited question:

The suspension (topology) and elementary examples

While the desuspension is also quoted/linked to the Wikipedia (withe refs given by Wiki). I also include a new note: "The desuspension is arguably firstly introduced in the cited text mentioned in H. R. Margolis (1983). Spectra and the Steenrod Algebra. North-Holland. p. 454." And the ref cited.

Follow people's comments, I modify to also ask the basic definitions:

Question: How do we define desuspension exactly? (Please see the comments below, people complain about the meanings of desuspension in Wikipedia is useless).

Are we able to have the desuspension acting on the topological space as the suspension does? Or do we only have the desuspension act on the spectra but not the space?

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Reopened

Please consider reopening this. It was closed originally as missing context. The OP provided some context in the comment and I have added a little bit more. Hope it is okay now.

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  • $\begingroup$ Wow, this question was through reopen review queue already four times. It makes me wonder what is the record. $\endgroup$ Sep 1 '18 at 1:04
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    $\begingroup$ I think it just how difficult it is to reopen a post using only the review queue. @MartinSleziak $\endgroup$
    – user99914
    Sep 1 '18 at 13:00
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    $\begingroup$ This one has 5 and is still closed. $\endgroup$
    – user99914
    Sep 1 '18 at 13:01
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Please consider undoing the duplicate votes here. The duplicate target is quite different as it asks for algorithms while this questions asks for easy ways to calculate determinant (so that careless errors are less likely to commit). The duplicate target is a poor question without context (though with some decent answers).

FYI: It failed the reopening review five times before, as of 3rd Sept. 2018.

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    $\begingroup$ Two points: 1. did you ask the user that put it on hold as duplicate about it? 2. It seems that the answer on the dupe target includes the answer on the original question. // Personally I think the answer given on this question is pretty much besides the point. And the question is very localised. Long story short. I'd delete post. $\endgroup$
    – quid Mod
    Sep 3 '18 at 11:59
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    $\begingroup$ @quid: I didn't (I thought this case is obviously wrong, and it had 4 reopen votes already). I agree that the question is localised and deletion is also a good choice. $\endgroup$
    – user99914
    Sep 3 '18 at 15:21
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Reopened

Please consider reopening the question A functional equation of a matrix that is placed On Hold. I have added my attempt at the cracking the problem if the lack of it was the reason for placing it on hold. The question itself is technically perfectly sound.

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    $\begingroup$ You should have just edited your previous answer, and not added another "answer." $\endgroup$ Sep 25 '18 at 7:07
  • $\begingroup$ @JoelReyesNoche: What do you mean? "Answer" to my question that I linked to? I did not post an answer to my own question. $\endgroup$
    – Hans
    Sep 25 '18 at 8:18
  • $\begingroup$ No, answer(s) on the present meta page, there should not be two for a single post on main. $\endgroup$
    – Did
    Sep 25 '18 at 9:31
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    $\begingroup$ I have removed the second post, since this one has been edited to indicate the reopening. I've also edited and deleted/undeleted, so whomever downvoted can undo their vote. $\endgroup$
    – Asaf Karagila Mod
    Sep 25 '18 at 9:56
  • $\begingroup$ @AsafKaragila: Oh, sorry. I did not know the first request was posted as I remembered closing the page rather than posting the "answer" as I needed to leave in a hurry earlier. I posted the second request thinking that the first request was not posted. This is the first time I post in the meta question site. $\endgroup$
    – Hans
    Sep 25 '18 at 10:59
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    $\begingroup$ Why this post says "reopened" when the linked question is still on hold at the moment? $\endgroup$ Sep 25 '18 at 12:31
  • $\begingroup$ @Martin, seems to have been a mistake, which I have rectified. $\endgroup$ Sep 25 '18 at 12:37
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    $\begingroup$ The thing that is missing in the current version of the question, more than work, is the source of the problem. Why do we expect it is true in the first place? $\endgroup$ Sep 25 '18 at 19:32
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Undeleted

The deleted answer for Show that there does not exist a unique stationary distribution. should be undeleted.

The question is about "existence of unique stationary measure", and the answer is concise and to-the-point.

"$(1,0,0,...,0)$ and $(0,0,...0,1)$ are two invariant distributions so uniqueness fails."

The existing answer shares the same idea with the deleted one, and it has passed a Low Quality Review.

(Edit: comment removed)

The deleted answer attracted an comment from a high-rep user during another Low Quality Review. However, by appealing to his/her tag score for the relevant tags (, , , , , etc) (and the contributing posts) and comparing them with those of the answerer, you'll have a better idea about their contributions to the site in those areas.

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    $\begingroup$ Oddly, the deleted answer was deleted, not through review, but by its owner. $\endgroup$ Oct 12 '18 at 11:51
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    $\begingroup$ You could ping the author on another of his posts, but I generally oppose undeleting a self-deleted post. $\endgroup$
    – user296602
    Oct 12 '18 at 15:14
  • $\begingroup$ The author has commented on the other answer, so he can be pinged there if necessary. This is an odd situation. $\endgroup$
    – Arnaud D.
    Oct 12 '18 at 17:34
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    $\begingroup$ Thanks for advice. I did ping the author. He can undelete this anytime he wishes. I ponder whether CRUDE is healthy, when some of its active users vote to delete short answers outside their familiar tags regardless of the quality of the answer. This isolated example shows that its malfunctioning in terms of quality control. $\endgroup$ Oct 12 '18 at 20:35
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    $\begingroup$ @GNUSupporter8964民主女神地下教會 This isolated example shows nothing about CRUDE, since the delete votes and the associated comment came from the review queue, and the answer was never mentioned in CRUDE. Besides, I'm not sure tag scores are necessarily an indicator of expertise : for example I have a low score in "integration" because I don't really like to compute integrals, but I still know the basics and I think I'm competent to judge the quality of reasonably simple answers on the topic. $\endgroup$
    – Arnaud D.
    Oct 15 '18 at 12:11
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    $\begingroup$ @ArnaudD. I see your points, but let me clarify my stance. 1. To be more precise, I would say that this post shows the influence of CRUDE participation on other posts (not listed on CRUDE). In a review queue, it's possible that one votes to close/delete within a few seconds. Given the amount of posts that they review every day (in/outside CRUDE), this example reminds us the adverse effect of their rare mistakes dispite their high reputation. 2. That's why I've added "contributing posts" inside the brackets. $\endgroup$ Oct 15 '18 at 12:47
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    $\begingroup$ If you read the posts instead of the scores, you'll find out the truth: a) there's no probability theory in his/her "probability theory" answers. These are mistagged (elementary) probability questions. b) His/her stochastic calculus tag score comes from a reference request question. By the way, he/she has removed his/her comment. $\endgroup$ Oct 15 '18 at 12:52
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Undeleted


I nominate Group algebra&algebra for undeletion so as to give the community enough time to judge this value of its answer, which is deleted due to the question asker's self-deletion.

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Reopened

(This is to break this request post into two)

I would like, if it is possible, remove the labels close-hold for some of next of my recent post Convergence of $\prod_{n=2}^\infty\left(1-\frac{1}{n^2}\right)^{i^n}$, where $i$ is the imaginary unit

I think that the answers are very interesting and very good, and I think that the questions are in the standard of questions of MSE. Any case many thanks.

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Undeleted, closed, deleted, re-undeleted, reopened

I nominate How to use derivatives to prove that $f(x)=2\cos^2\left(\frac{\pi }{4}-\frac{x}{2}\right)-\sin \left(x\right)=1$? for undeletion since OP has self-deleted his/her question after receiving an answer. This is unacceptable on Math.SE.

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  • $\begingroup$ It's better to write self-contained posts. In the current case for the moment there was no big risk, but it's still not ideal. $\endgroup$
    – quid Mod
    Nov 8 '18 at 19:42
  • $\begingroup$ @quid Thanks for your intervention. I'll correct this now. $\endgroup$ Nov 8 '18 at 19:44
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    $\begingroup$ I don't understand why this question was closed and deleted by users. It is not unclear what they're asking. (It may not be true, but it is certainly clear!) $\endgroup$
    – user1729
    Feb 5 '19 at 11:28
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    $\begingroup$ @user1729 Some users don't like wrong info in the question. They have standards so high that $D(\sin x)$ and the RHS of $f'(x)$ doesn't please them. It's possible that they used their power to get rid of this question. $\endgroup$ Feb 5 '19 at 11:37
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    $\begingroup$ It's definitely not unclear—the questioner gives the problem, shows what they did, and asks what's wrong with it. I think it could do with an additional answer which makes crystal clear what the error was and how the (interesting!) technique works, and reopening would allow someone to post that. $\endgroup$
    – timtfj
    Feb 5 '19 at 12:06
  • $\begingroup$ @timtfj It's now reopened. $\endgroup$ Feb 5 '19 at 16:04
  • $\begingroup$ @GNUSupporter8964民主女神地下教會 At least it's clear that their expectations could never rise to match the size of your assumptions... $\endgroup$
    – Lord_Farin
    Feb 6 '19 at 17:34
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Please reopen The characteristic polynomial of $A$ is $x^n$ if and only if $\text{Tr}(A^i)=0$ for all $1\le i \le n$. due to the reasons listed in OP's comment. The first duplicate matches half of the question. The hypothesis of the second duplicate is different from the first one.

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Reopened

Please consider reopening Is there an injective homomorphism from $S_4$ to $GL(2,C)$ because the question asker has shown efforts in solving the problem, and it has a score of 4 with an accepted answer of score 3 from a 25k user.

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    $\begingroup$ Answers should not be used to judge whether a question should be reopened. But in this case, the asker did include sufficient context, so that's irrelevant. $\endgroup$
    – user21820
    Mar 25 '19 at 9:44
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    $\begingroup$ "Answers should not be used to judge whether a question should be reopened." That's your opinion, @user – it is not a rule, or even a consensus. $\endgroup$ Mar 25 '19 at 11:51
  • $\begingroup$ @GerryMyerson: It is your comment that is an opinion; it's not even a consensus. Both this and this clearly demonstrate a consensus that "bad questions beget bad questions" and "closed questions can and should be deleted even if the have good but standard answers; only truly great answers should rather not be deleted". $\endgroup$
    – user21820
    Mar 25 '19 at 12:37
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    $\begingroup$ @user21820 Thanks for your comment. I'll be aware of that next time. $\endgroup$ Mar 25 '19 at 16:39
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    $\begingroup$ @user, your quote refers to deletions, not closure/reopening; what's more, the part that says "only truly great answers should rather not be deleted" plainly contradicts your assertions that "answers should not be used to judge...." $\endgroup$ Mar 25 '19 at 21:51
  • $\begingroup$ @GerryMyerson: Firstly, if a bad question ought to be deleted, then obviously it should not be reopened. Secondly, it would be real silly to assume that everything I say is to be taken completely rigidly with no room for exceptions; from my very first comment I meant that as a rule of thumb answers should not be used to judge... $\endgroup$
    – user21820
    Mar 26 '19 at 5:43
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    $\begingroup$ @user, I'm not sure how I am to tell which things you write are to be taken completely rigidly, and which allow room for exceptions. Maybe we could work out some signal, so in the future I'll be able to tell. $\endgroup$ Mar 26 '19 at 8:51
  • $\begingroup$ @GerryMyerson: Usually, I try to be precise, using words like "typically" or "almost always" but, you know, that that is very tiring to do in every single statement. How about you just assume that there may be rare exceptions unmentioned unless I use the word "absolutely"? I don't think anyone is 100% precise all the time anyway, so that works for interpreting others' comments too. $\endgroup$
    – user21820
    Mar 26 '19 at 8:59
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Reopened

Please undelete Generalization of the fundamental theorem of duality since the question asker has deleted his/her own question shortly after receiving an answer. This is an abuse of the system.

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Undeleted and reopened

Please reopen Proving the greatest lower bound. because OP has self-deleted his/her own question after I've answered this question. This inhibits the community from viewing the question.

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Reopened

Please consider reopening the following question:

Maclaurin Series Expansion of $\ln(1+\sin x)$

The details which were included afterwards contradict the reason of closure. Moreover they indicate that the OP has as fundamental erroneous understanding of the concept of a MacLaurin Series - and how to derive them -rather than the unwilligness of showing his own efforts. In my opinion the question should be reopened.

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Reopened

Please reopen Am I have right answer about dual problem? since OP has responded to the comment and used MathJax to type out the math. In the revised version, OP has attempted to construct the dual program of the primal program, so it's OK for reopening.

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Reopened

This question was closed as "unclear what your asking":

Show that the order of an element g is well-defined

The issue and its resolution can be seen in the comments to the thread. A brief summary is: I and others didn't understand the question (possibly others influenced by me), but I understand it now. Its fine in its current form.

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    $\begingroup$ @Brahadeesh How is it closed as a duplicate? I didn't see it when I clicked into the question... $\endgroup$ May 2 '19 at 8:21
  • $\begingroup$ @YuiToCheng Terribly sorry, I could swear I saw it was closed as a duplicate. I'm reverting my edit. Don't know how that happened. :/ I'll be more careful next time. $\endgroup$
    – user279515
    May 2 '19 at 8:24
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Reopened

I nominate A question of arithmetic regarding erection cost of a structure. for reopening since the question asker has shown his/her work in solving the problem.

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    $\begingroup$ I agree, but think they should also be encouraged either to edit what they said in comnents into the question, or to give someone else permission to do so. $\endgroup$
    – timtfj
    Feb 5 '19 at 12:20
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Undeleted, put on hold, deleted

Please undelete https://math.stackexchange.com/q/3209899/290189 since OP self-deleted her own post after receiving an answer. This is an abuse of the system.

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    $\begingroup$ I undeleted it. Please try to use correct terminology. Undelete and reopen are not the same. $\endgroup$
    – quid Mod
    May 2 '19 at 7:33
  • $\begingroup$ @quid Thanks for your undeletion and reminder. I'll be aware of that next time. $\endgroup$ May 2 '19 at 11:49
  • $\begingroup$ Question is currently on hold. $\endgroup$ May 3 '19 at 3:06
  • $\begingroup$ @GerryMyerson Thanks for notification. Status updated. $\endgroup$ May 3 '19 at 6:52
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Undeleted and closed as duplicate

Choose 2 good batteries out of 8 (4 bad 4 good) was deleted by its author, after an answer was posted. This is considered to be an abuse of the system.

Full disclosure: the only answer posted before the self-deletion was mine.

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    $\begingroup$ Currently closed as a duplicate. $\endgroup$ May 3 '19 at 3:07
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Undeleted

Please undelete A probability to 6 in a dice since OP has self-deleted his/her own question after receiving an answer. This isn't fair to the answerer because his answer deserves review from the community.

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Both undeleted, second link redeleted

Please undelete A contradiction in calculating the legendre symbol and https://math.stackexchange.com/q/3224172/290189 because OP's have self-deleted their question after receiving an answer from others.

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    $\begingroup$ I undelete both. At least for the one, it'd also have been possible to let it deleted as it was a relatively simply oversight. It's not clear there is any value in keeping that around. But there is also no harm, so. $\endgroup$
    – quid Mod
    May 14 '19 at 15:31
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    $\begingroup$ Please adhere to the "one request per answer" format; having multiple requests in the same answer makes it difficult to track changes. $\endgroup$
    – user279515
    May 15 '19 at 8:03
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    $\begingroup$ @Brahadeesh Thanks for your comments. I'll beware of this practice next time. $\endgroup$ May 15 '19 at 9:00
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Undeleted.

Please undelete Bounding the number of edges in a graph satisfying a certain property since the question asker has self-deleted his/her question after receiving an answer.

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Undeleted

Please consider undeleting the question:

Intersection of Normal closure and Center

The OP deleted question after answer was given.

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    $\begingroup$ I undeleted, but maybe write a proper answer next time. $\endgroup$
    – quid Mod
    Jun 4 '19 at 13:39
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    $\begingroup$ @quid I like hints (plus its a pretty straightforward question - I spent a while debating whether to just leave it at "kill $t$"). $\endgroup$
    – user1729
    Jun 4 '19 at 13:43
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    $\begingroup$ Then maybe start a Q&H site. This is Q&A site. $\endgroup$
    – quid Mod
    Jun 4 '19 at 18:09
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    $\begingroup$ @quid the hint was clearly sufficient for the OP - they wouldn't have deleted their question otherwise! :-) $\endgroup$
    – user1729
    Jun 4 '19 at 19:00
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    $\begingroup$ That's pretty irrelevant to my point. Further, If "sufficient for the OP" is your main concern, why do you bother us with an undelete request? // The way that hint was orginally presented to me personally made it pretty confusing. The new format is in a way better. But, frankly I just despise that form. I know it's popular but I think it's terrible style. $\endgroup$
    – quid Mod
    Jun 4 '19 at 22:55
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Reopened

Please consider reopening Does knowing the surface area of all faces uniquely determine a tetrahedron?. This is a very natural and self-motivating question that does not need any additional context, and it's gotten several great answers.

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Reopened

Please consider reopening this post, suitably narrowed since originally closed as "too broad":

Book recommendation... Linear Programming for self-study

I don't believe there has been another such request, and under my pestering the OP has provided context for what sort of self-study they've previously undertaken.

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Undeleted, reopened

Please consider undeleting and reopening this post:

How does one prove the inequality $1+|x|\le (1+|y|)(1+|x-y|)$?

OP clearly indicated the context of his/her question: it is from a proof in Wolff's lecture notes on harmonic analysis.

[Added upon request: this post has also been edited into a (more) decent one.]

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    $\begingroup$ If you edit a post significantly indicate so in the request. I have no problem with it by itself, but since such actions are quite frequently overlooked by observers it can lead to the false impression that a relatively decent post was deleted. $\endgroup$
    – quid Mod
    Jul 8 '19 at 14:07
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    $\begingroup$ @quid If someone else (not OP) edits OP's question to make the question 'fit' on MSE, then does the question gets reopened or undeleted always? $\endgroup$
    – user486983
    Aug 9 '19 at 18:48
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    $\begingroup$ I said different by OP because sometimes the question is old, say 2015, and OP hasn't visit MSE since 2015. $\endgroup$
    – user486983
    Aug 9 '19 at 18:51
  • $\begingroup$ @Isa I'd say yes for the most part (although what 'fit' mean of course leaves room for interpretation). That said, some users do not like the practice, thus there can also be push back. But in abstract and as a general principle (there can be exceptions in special cases) if a question is a good fit for math.se we usually keep it around no matter how it arrived in this state.// If you want to discuss this in more detail please open a separate thread. $\endgroup$
    – quid Mod
    Aug 9 '19 at 19:32
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Undeleted, reopened, closed as duplicate

Please consider undeleting this well-received (33 net upvotes) question under the tag of probability: Probability of drawing the Jack of Hearts?

There are useful discussion and several good answers, one of which has 77 upvotes.

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  • $\begingroup$ The question was originally closed as "off-topic: lacking context" (which, in my opinion, was not unreasonable). It is, however, also a duplicate of other questions, as indicated by the comments. If it is to remain on the site, it should be properly linked via a dupe closure. $\endgroup$
    – Xander Henderson Mod
    Aug 19 '19 at 17:38
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    $\begingroup$ At most "similar" or "related". Not a duplicate. $\endgroup$
    – user9464
    Aug 19 '19 at 17:54
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    $\begingroup$ I'm sorry, but what? The question above asks us to determine the probability of drawing a Jack from a deck of cards from which an unknown card has been removed. One of the two questions in the dupe target asks for the probability of drawing an Ace from a deck of cards from which an unknown card has been removed. These are precisely the same question, and answers to the older question completely answer the newer question! How is this not a duplicate? $\endgroup$
    – Xander Henderson Mod
    Aug 19 '19 at 18:32
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    $\begingroup$ Mr. Henderson, I have a very narrow definition of "duplicate" from yours. No need to be sorry. And of course you do have the right to vote it as a duplicate. $\endgroup$
    – user9464
    Aug 19 '19 at 18:34
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    $\begingroup$ I marked it as a dupe. That said @Xander I think the difference is slightly larger than you make it look. This question is about one specific card (a jack of hearts), while the dupe is about a group of cards (an ace). I still think that is a duplicate. // Since the comment on main got auto-deleted I'll add that two users other than me had mention it as a dupe. Thus, it was at least a trilateral closure. :-) $\endgroup$
    – quid Mod
    Aug 19 '19 at 18:54
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    $\begingroup$ @quid Indeed, I had missed that. That said, as you note, the distinction is not fundamental. Thank you for handling it. $\endgroup$
    – Xander Henderson Mod
    Aug 19 '19 at 19:39
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Reopened


The question Problem with sum of projections was incorrectly marked as a duplicate of Orthogonal projections with $\sum P_i =I$, proving that $i\ne j \Rightarrow P_{j}P_{i}=0$. The latter question has the additional hypothesis that the projections are self-adjoint (i.e., orthogonal projections) which allows for some rather different proof methods. Indeed, none of the four answers to the second question solve the first question.

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